The present invention relates to a method of repairing a ROM array and to a memory circuit.
The memory pattern programmed into a read-only memory (ROM) cannot be changed and therefore it is essential that the program to be stored in it is correct and free from errors. Of course, both manufacturing and programming defects do arise during the fabrication of such memories.
The yield of the fabrication process can be improved if ROM'S including defects can be repaired. However, this is only economic if the ROM can be made to be repairable simply and without complex circuitry.
In connection with random access memories, it has been proposed to provide spare memory capacity which can be used in place of defective locations in the memory. See, for example,
British Pat. No. 1455716 PA1 British Pat. No. 1398438 PA1 British Pat. No. 1311997 PA1 British Pat. No. 1550675 PA1 British Pat. No. 1528100 PA1 British Pat. No. 1488279 PA1 British Pat. No. 1531528 PA1 British Pat. No. 1346219
Attempts have also been made to provide redundancy for ROM arrays, as evidenced by:
but, in general, these proposals involve complex and therefore expensive circuits.
None of these specifications concerns itself with the problem as to how the data, which will be erroneously represented in the defective locations of the ROM, can be easily determined so that it can be stored in the spare memory capacity.